1. Field of the Invention
The present invention relates generally to devices and methods for treating spinal disorders and more specifically to an intervertebral device for aligning and maintaining the relative position of two or more adjacent vertebrae as well as to contain graft material to facilitate immobilization of the vertebra through fusion to eliminate the pain caused by abnormal motion.
2. Description of the Background
Degeneration of the intervertebral discs and the concomitant instability and translocation of the vertebra is a common cause of back pain and may result from a variety of problems including congenital deformity, age related degeneration, osteoporosis, tumor and disc herniation as a result of trauma. Disc degeneration, for whatever reason, results in compression of the spinal nerve roots resulting in pain. Palliative care is often successful in mild cases but more extreme or degenerative cases may require a surgical approach to stabilize the joint and relieve pressure.
A number of surgical approaches have been developed with varying degrees of success depending on the cause and severity of the damage. A ruptured disc impinging the nerve root may be partially excised to relieve pressure. In such a case the adjacent vertebra may be further fixated using rods, screws and plates in an attempt to stabilize the spine and delay or prevent further degeneration. Patients undergoing such excisions and fixations however, often require subsequent procedures to address recurrent pain. In many case such subsequent procedures include fusion. Spinal fusion, or spondylosyndesis, is a surgical technique used to combine two or more vertebrae utilizing supplementary bone graft tissue in conjunction with the body's natural osteoblastic processes to eliminate relative movement as a source of pain. A variety of approaches to fusion are available including posterior fusion, postero-lateral fusion and anterior or posterior interbody fusion.
In the more traditional posterior fusion approach, performed in conjunction with partial excision of the ruptured disc, growth is induced between the bony vertebral laminae to fix the position of the vertebra. In the postero-lateral fusion method bone growth is induced to join the transverse processes to prevent motion between the adjacent vertebrae. However, both posterior and postero-lateral fusion tend to cause bony overgrowth leading to nerve root compression and pain by spinal stenosis. This, coupled with other risks, limitations and disappointing fusion success rates have caused surgeons searching for alternate fusion means to develop interbody fusion techniques.
Interbody fusion techniques involve complete excision and replacement of the soft disc with autograft material harvested from the patient, prepared allograft from a donor source or, more recently, bone morphogenic protein. Most commonly performed in the lumbar region, the procedure can be accomplished from an anterior approach (Anterior Lumbar Interbody Fusion or ALIF) or a posterior approach (PLIF). In either case the procedure attempts to reconstruct the normal anatomic relationships between the bony and the neural structures and has many advantages. Specifically, weight bearing through a solid bony fusion mass between vertebral bodies relieves the mechanical pain of the traditional unstable degenerative disc and generally prevents long term disc collapse or further degenerative changes. The complete disc excision prevents recurrent herniation of the same degenerated disc.
Successful fusion results in a contiguous growth of bone to create a solid mass that will unite the vertebra. When fusion graft material is first placed it is soft and movable and lacks cohesive strength and is therefore incapable of remaining in position or carrying any load without assistance. A variety of appliances have been developed that attempt to hold the vertebrae to be joined still relative to one another under normal spinal activity and daily stress in order to allow the fusion process to occur over the 18-24 month period generally required. Such appliances, often referred to as interbody cages, provide a mechanically rigid scaffold in which the graft material may be placed.
Cage designs vary widely but generally fall into three categories. Horizontal cylinders (1) are generally made from titanium and inserted by either the posterior or anterior approach into complimentary holes bored into the intervertebral space. They can be placed by open or minimally invasive techniques. U.S. Pat. No. 5,026,373 to Ray, et al. discloses a cage of this design that includes a perforated threaded exterior surface that can be screwed into place between the vertebra and packed with bone material. Bone growth through the perforations and into the cancelous bone of the vertebra exposed by the insertion results in the desired fusion.
A second design is in the form of a vertical cylinder or ring (2). Often referred to as a Harms cage, vertically cylindrical cages are also usually made from titanium and can be cut to length as desired so as to span larger segments of the lumbar spine. End caps are employed to prevent subsidence into the cancelous bone although this design suffers, as a result, from a requirement that its central void be packed with graft material prior to insertion. Due to its sharp edges it is most commonly inserted by open techniques. U.S. Pat. No. 5,989,290 to Biedermann et al, et al. discloses a cage of this design.
A third design form is the open box cage (3). Constructed of carbon, titanium or bio-compatible non-metallic materials, this design can be formed for an anatomical fit or to recreate the normal lumbar lordosis. Openings in the box walls permit graft material contained therein to contact the vertebral bone. Some designs utilize a single large cage. Alternately, a pair of smaller cages is utilized which can be inserted posteriorly using minimally invasive techniques. U.S. Pat. No. 6,241,769 to Nicolson et al, et al. discloses a box form cage having a central void having an open top and bottom and a dovetail system for structurally attaching the device to the adjacent vertebra which are prepared by cutting cooperative channels into their surfaces.
Cages provide enhanced mechanical stability prior to fusion, maintain the intervertebral disc height and ultimately provide a high rate of successful fusion. The ideal cage should rigidly immobilize the spine in all directions, be strong enough to withstand repeated loadings, have a modulus of elasticity similar to that of cortical bone. It should also be easy to insert by open or minimally invasive methods, resist subsidence, translation or retropulsion and be clinically effective. Cage designs further must balance the competing priorities of being small enough to be inserted through the incisions of minimally invasive techniques while also being large enough to fill a significant portion of the interbody space and present a significant area to the vertebral surface in which graft material can be inserted and retained to promote growth.
It would be therefore an improvement in this art to provide an interbody fusion cage for facilitating vertebral fusion and thereby eliminating spinal back pain caused by ruptured or degenerated vertebral discs which overcomes the deficiencies of prior known devices. Thus, it is an object of the present invention to provide an interbody fusion cage of open form design that can easily be placed in the evacuated interbody space to constrain relative vertebral motion and which can subsequently be secured again translation and retropulsion. It is a further object of the present invention to provide an interbody fusion cage that is sufficiently robust so as to withstand the forces imposed by normal daily activity on the part of the patient and which is clinically effective it retaining osteoconductive or osteoinductive material so as to facilitate fusion.